Cool. A couple of advisory notes regarding your plans.
1. Multiple theremins - theremins are by nature solitary beasts. The distance at which they can affect each other is larger than the playing field. Typical effects of this are somewhat similar to ring modulation - I suspect (in my non-electronic engineer way) they are a result of minor variations in their radio frequencies. I am not familiar with the theremaniacs devices, but strategies which can ameliorate this include:
- ensuring there are no ground loops - theremins benefit from being earthed, but not from sharing a common earth. When working with theremin ensembles one way of avoiding this is to power the instrument via a long extension cable (a few metres) which has the earth disconnected at the end furthest from the instrument. Naturally you should confirm the safety of this procedure with an expert.
- putting a large conductor between interfereing theremins - such as the player's body (probably not applicable for your intended use),
- ensuring sufficient distance between theremins. However much that is for your bollards.
2. Dancers. The reason the terpsitone (Theremin's room sized theremin, played by dancing) was not a success was that they could not find dancers who could successfully combine dancing and playing. To be fair this was a rather different situation - the music was classical and the theremin sound not post-processed.
I appreciate that you have designed your bollards to be "easy to play" but that does not mean that there will not be scope for better playing to be achieved from practice and experimentation. The essence of theremin playing is a cybernetic feedback loop - the sound varies in response to movements, the player varies his movements in response to the sound. So - as I was advised when I first started - listen, listen, listen! That's the key. :-)